Within the last few decades, the field of synthetic biology and genetic engineering has developed exponentially. However, biologists have lacked a cheap and easy tool to make direct edits into desired sequences or parts in an organism’s genome.
The molecular scissors of bacteria
This changed in 2014 in a paper published by Jinek et al. The team described the clustered regularly interspaced palindromic repeats (CRISPR) and its CRISPR-associated protein (Cas) as a programmable tool for genome editing. This cut and paste-like system is derived from the natural adaptive immune system of bacteria and archaea.
In these tiny organisms, Cas9 is able to identify and cut (aka disrupt) specific foreign DNA fragments. Their immune mechanism is activated when short DNA fragments of invading viruses or bacteria integrate into the host genome and form CRISPR arrays. These are used to make a complementary CRISPR RNA guide sequence, that base pairs with the target site of the invader and guide the DNA-shredding Cas9 enzyme to mutate it.
Programmable genome editors
These Class II Cas complexes, such as the DNA targetting Cas9 and the RNA targetting Cpf1, have become useful tools for genome editing. This is due to their RNA guided specificity and DNA-cutting activity. The RNA guide can be artificially simplified into a single guide RNA (sgRNA). The sgRNA comprises a 20 nucleotide target binding sequence and a hairpin structure required for Cas enzyme recruitment and target site recognition. The Cas9-sgRNA complex can mutate and knockdown target genes, or create dual breaks for gene insertion through homology directed repair.
Furthermore, the CRISPR toolbox has been extended to epigenetic editing. This includes fusing gene activation or inhibition effector protein domains to a deactivated Cas9 protein, which binds to a target sequence but without making a cut. This can enable editing with less off-target effects and without permanently damaging the DNA strand.
What can we do with CRISPR?
CRISPR has wide application in biotechnology, cell therapy, xenotransplantation, and functional genomic studies. However, CRISPR/Cas tools require more efficient delivery methods and improved specificity before they can hit the clinics.